1. Field of Invention
The present invention relates to a balun-transformer composed of a balun transition and an impedance matching circuit.
2. Related Art
The design of microwave circuits often need to convert from balanced lines to unbalanced lines and, at the same time, solve the problem of impedance matching. They are usually made according to experience and experiments. The most popular circuits with balun-transformer include the push-pull amplifiers and the sampling phase detectors.
Generally speaking, the following methods are often used to solve the above problems.
Coaxial-Line Type Balun-Transformer: Because of the good performance in power handling, it is often used in high power components. The coupling length is one fourth the wavelength at the usage frequency. The coupling length is already greater than three inches between 400 MHz and 500 MHz. It is too large to be realized in circuits. This type of transformer needs very good soldering technology to fix and ground it.
Ferrite-Toroid-Core Type Balun-Transformer: Its volume is smaller and can be put into the SMT package. The soldering technology is easier but the power handling capability is poor. Most of all, it has a higher cost.
In the actual design process of the disclosed balun-transformer, an odd mode analysis is first employed to design accurately and a commercial microwave software is then used to simulate.
The disclosed balun-transformer has two preferred embodiments. The first preferred embodiment chooses a conductor-backed coplanar waveguide (CBCPW) to conductor-backed coplanar stripline (CBCPS) transition to convert balanced line (CBCPW) to unbalanced line (CBCPS). An impedance matching circuit is designed following this CBCPW to CBCPS Transition. This kind of balun-trasfomer can be used from several MHz to 10 GHz.
A second preferred embodiment of the invention is a simplified form of the previous embodiment, wherein only the CBCPS is used. One node of its input port is grounded. The signal of the CBCPS output nodes is kept at a 180xc2x0 phase difference. The impedance matching circuit is further employed. This simplified design can be used between several MHz and 2 GHz.